Comparison of ADV measured near-bed orbital speed and latter derived from wave gauge measurements at intermediate water depths

Abstract

The aim of this study is to assess the applicability of the pressure wave gauge to measure wind waves at transitional water depths. In this study, a rather uncommon method is proposed to check the validity of the linear wave theory. Namely, we compare the wave induced near-bed velocities measured directly with the Acoustic Doppler Velocimeter (ADV) to those calculated from wave gauge measurements. A subsurface mooring station equipped with an ADV and a pressure wave gauge was deployed in Keibu Bay, a small coastal environment in the southern Gulf of Finland, at a depth of 7 m in June 2010. Among long period oscillations, i.e. currents, the ADV allowed measuring wind wave induced orbital velocities. The ADV was set to measure orbital velocities 27 cm above the bottom at 2 Hz, while the pressure sensor of the wave gauge was 5 m from the bottom and measured pressure at 4 Hz continuously. After removing turbulent velocity from the ADV measurements, the combined horizontal spectrum was calculated and the bottom orbital speed was derived. The surface elevation spectra were estimated from the pressure measurements and the near-bed orbital speed was calculated from the wave spectra. The measurement campaign was characterized by diverse wave activity. It presents growing wind seas, fetch limited, swell and mixed sea states. The maximum significant wave height was 1.12 m. The wave data obtained were examined for their region of validity in the framework of Stokes' and Airy's wave theories. 80 % of data were in the range of Stokes' wave theory. The comparison of the ADV measured near-bed orbital speed and the data derived from wave gauge measurements showed remarkable agreement. The correlation coefficient was 0.98 and the root-mean-square error for orbital speed was 1.2 cm/s. During the largest wave event, the measured near-bed orbital speed reached 25.8 cm/s over a 5-minute period, and the calculated speed was 25.2 cm/s. In conclusion, the pressure data measured in coastal seas can be transformed into surface waves via the linear wave theory.